The present invention is directed to a process for the production of molded block fuel elements for gas cooled high temperature reactors by hot molding of a granulated graphite material together with fissile and fertile coated fuel particles using stearic acid as lubricant and a suitable organic compound as air displacement agent.
The molded block fuel element for high temperature reactors for short is also called monolith, is a hexagonal prism 700 to 1000 mm high with a width over the flats of, for example, 360 mm and a weight of, for example, about 150 kg. The monolith consists of a substantially fine crystalline graphite matrix of high heat conductivity, fuel regions with the same matrix and cooling channels. The fuel regions contain the fuel in the form of coated particles which are embedded in the graphite matrix. According to the construction of the fuel element the number of fuel regions customarily is 138 to 216 and the corresponding number of the cooling channels is 72 to 108. In contrast to a bored block and mechanically worked graphite fuel elements with loosely filled fuel inserts the fuel regions of the monolith are well bonded to the remaining graphite matrix so that both parts of the block, that is the fuel containing graphite matrix and the fuel free zone form a monolithic structure. Therewith a high cooling gas outlet temperature is reached at relatively low fuel temperature. Consequently the fission products release is reduced and therefore the monolith is not only suitable for steam cycle plants but also for direct cycle plants with helium turbines and particularly for nuclear process heat reactors. At the same time it fulfills the requirement of a fuel element with increased heavy metal content for a high temperature reactor-high converter since the fuel zones in contrast to those in bored blocks can be enlarged without thereby weakening the block structure. (Further advantages of the monolith are described in German Pat. No. 1,902,994.)
The monolith is generally produced from granulated graphite powder containing binder resin and coated fissile and/or fertile fuel particles by molding. As binder resins there are suited poly condensation products with the highest possible softening point as for example polyester (i.e., unsaturated polyesters) and epoxy resins (e.g., (diphenylepoxychlorohydrin resins). Preferably, however, there are used novolak type phenol-formaldehyde resins. As graphite powder there can be used either natural graphite or synthetic graphite or a mixture of both types of graphite. The principle for production is described in German Pat. No. 2,104,431 and related Hrovat U.S. application Ser. No. 577,054 filed May 13, 1975 and German Pat. No. 2,234,587 and related Huschka U.S. Pat. No. 3,985,844. The entire disclosures of the Hrovat U.S. application and Huschka U.S. patent are hereby incorporated by reference and relied upon.
A series of requirements are placed on the block fuel elements. The outer dimensions of the hexagonal prism as well as the diameter and the positions of the numerous cooling channels and fuel regions produced by pressing only permit a deviation of several tenths of a millimeter together and for the longitudinal axis of the block from the nominal values. Since the coating of the fissile and fertile particles must remain intact in the production of the fuel element the molding pressure is also limited.
In spite of this limitation of the molding pressure the graphite matrix must have high geometrical density, good strength properties, high heat conductivity, the least possible modulus of elasticity, a small thermal expansion coefficient and a good crystalline arrangement. Besides it must not have disadvantageous property gradients in either the axial or radial block direction. All of these properties must be so adjusted to each other that the sum of the stresses occurring in the fuel element consisting of primary stresses (caused by the handling in loading the reactor), thermal stresses and radiation induced stresses during the total residence time in the reactor do not endanger the mechanical integrity of the block.
Besides the reactor operation requires that there be produced blocks with different fissile and fertile material loadings. In spite of different loading which strongly influence the amount of shrinkage in the heat treatment, the fuel element must come out without change in dimension.
Furthermore, a smooth cooling channel surface is required of the fuel element in order to obtain a low pressure drop of the helium gas during the reactor operations.
According to the preciously known molding processes there could not be produced block-fuel elements completely meeting requirements. The air contained in a loose charge before the pressing is disadvantageous. It is compacted to the center of the block during the course of the molding process and subsequently is pressed in there. The resilience of this compressed air in the initial phase of the subsequent carbonization leads to a change in shape and a weakening of the block structure, which in this phase is still very impermeable with relatively low strength. After the carbonization the block consequently exhibits impermissible property gradients toward the middle as well as a barrel shaped swelling of about 1 mm.
The construction of the fuel elements for a power reactor provides for the purpose of optimization of the use of fuel according to the block position and the residence time of different heavy metal loadings per fuel element in the period of reactor operation. The amounts of heavy metal per block-fuel element for a reactor differ very strongly from each other and customarily lie in the range between a minimum charge of 3 kg and a maximum charge of 20 kg per block. If one proceeds from the point that uranium and thorium are present in the form of separate fissile and fertile coated fuel particles then the above-mentioned amounts of heavy metal correspond to a particle content of 5 - 32 kg. Since the coated particles in contrast to the graphite matrix do not shrink during the heat treatment it is obvious that after the carbonization the highly loaded blocks in comparison to the low loaded blocks must have a larger diameter. The measurements show that in the required range dimension deviations of the block diameter occur up to about 2 mm. In order to compensate these deviations according to the present state of the fabrication for each heavy metal loading its own set of tools when necessary which besides increased investment costs had as a result additional operating costs with the change of tools.
It was therefore the object of the invention to avoid the described technological difficulties and to make possible an economical manufacture of gradient free block-fuel elements with different loadings and very good accuracy in size in the same tool, independent of fissile and fertile fuel loading of the block without endangering the mechanical integrity of the coated fuel particles.